In an IB Biology diagram of a dicotyledonous plant stem cross-section, identify the epidermis, cortex, phloem, and xylem. The xylem and phloem occur together in vascular bundles, with phloem nearer the outside and xylem nearer the centre.
This is SL and HL content in subtopic B3.2, Transport. Each tissue has a position and function that reflects its structure.
| Tissue or region | Position in the stem | Main function |
|---|---|---|
| Epidermis | Outermost layer | Protects the stem and reduces uncontrolled water loss. |
| Cortex | Between the epidermis and vascular bundles | Provides support and may store substances. |
| Phloem | Outer part of each vascular bundle | Translocates sap containing dissolved organic compounds, including sugars and amino acids. |
| Xylem | Inner part of each vascular bundle | Transports water and mineral ions from the roots and provides mechanical support through lignified walls. |
The vascular bundles are arranged around the stem rather than forming one central mass. This arrangement helps the stem resist bending while distributing transport tissues throughout it. In each bundle, xylem vessels are positioned toward the centre of the stem, while phloem is positioned toward the epidermis.
A common misconception is that a vascular bundle is itself one tissue. It is a region containing multiple transport tissues, principally xylem and phloem. Another common error is reversing their positions: remember phloem outside, xylem inside.
For an IB exam plan diagram, draw clear single lines without shading, show the relative distribution of tissues rather than individual cells, and label epidermis, cortex, vascular bundles, xylem and phloem. If asked to annotate, add each tissue’s transport or protective function; labels alone do not satisfy the command term annotate.
